1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and, more particularly, to a method of fabricating a complementary metal oxide semiconductor (CMOS) type semiconductor device having dual polysilicon gates.
2. Description of the Related Art
The CMOS type semiconductor device is a device which has a P-channel MOS (PMOS) transistor and an N-channel MOS (NMOS) transistor disposed on one semiconductor substrate to perform complementary operations.
One method for implementing the CMOS type semiconductor device is a single gate technique. The single gate technique uses N-type doped polysilicon gate electrodes for the PMOS and NMOS transistors. This single gate technique has the advantage of low cost, however, adjusting the operating voltage Vt of the PMOS transistor is difficult.
Another method for implementing the CMOS type semiconductor device is a technique that uses a metal material for the gate of the transistor instead of polysilicon since the metal gate has better conductivity. However, the metal gate degrades the gate insulating layer due to metal ions, and this makes it hard to adjust the operating voltage Vt due to its fixed work function. As described above, in order to implement the CMOS type semiconductor device having the NMOS transistor and the PMOS transistor on a single chip, the operating voltage Vt of the NMOS transistor should be different from that of the PMOS transistor. As a result, a metal gate used in the NMOS transistor region should be different from that used in the PMOS transistor region, which complicates the manufacturing process.
A method of forming a CMOS type semiconductor device using a metal gate is disclosed in U.S. Pat. No. 6,468,851 B1 entitled “Method of fabricating CMOS device with dual gate electrode” to Ang, et al.
According to Ang, et al., an N-type polysilicon gate electrode is formed in the NMOS transistor region and a metal gate electrode is formed in the PMOS transistor region. Copper, aluminum, titanium nitride, or tungsten is used as the metal gate electrode. For example, in order to form a copper gate electrode, a dedicated copper facility is required and a technique that prevents the copper from diffusing is also required. This complicates the manufacturing process.
Another method for implementing the CMOS type semiconductor device is a dual polysilicon gate technique. The dual polysilicon gate technique is a technique which allows an N-type polysilicon gate electrode to be formed in the NMOS transistor region and a P-type polysilicon gate electrode to be formed in the PMOS transistor region.
FIGS. 1 and 2 are cross-sectional views illustrating some of the processing steps in fabricating a CMOS type semiconductor device having dual polysilicon gates in accordance with the prior art.
Referring to FIG. 1, field isolation layers 2 are formed on the semiconductor substrate 1 to define active regions. A P-type well 3 and an N-type well 4 are formed within the active regions. An NMOS gate insulating layer 7 and an NMOS gate electrode 9 are formed on the P-type well 3 which are sequentially stacked to cross over the P-type well 3. A PMOS gate insulating layer 8 and a PMOS gate electrode 10 are formed on the N-type well 4 which are sequentially stacked to cross over the N-type well 4. In this case, the gate electrodes 9 and 10 are formed of N-type doped polysilicon. Lightly doped impurity regions 5 and 6 are formed in upper regions of the P-type well 3 and the N-type well 4. Subsequently, spacers 11 are formed on sidewalls of the gate electrodes 9 and 10.
Referring to FIG. 2, N-type impurity ions are selectively implanted into the P-type well 3 to form NMOS source and drain regions 13. A photoresist pattern 15 covering a top surface of the P-type well 3 is formed on the semiconductor substrate 1 to expose a top surface of the N-type well 4. Subsequently, P-type impurity ions such as boron (B) are selectively implanted into the semiconductor substrate 1 using the photoresist pattern 15 as an ion implantation mask to form PMOS source and drain regions 18. At the same time, P-type impurity ions are also implanted into the PMOS gate electrode 10 to form a P-type doped PMOS gate electrode 10′.
Electronic products using semiconductor devices are in pursuit of small size, lightweight, and high performance, therefore semiconductor devices need to have high integration density, low operating voltage Vt, fast operating speed, and low power consumption. For high integration density, a transistor's gate, source and drain junctions, and interconnects should be made as small as possible. However, shrinking the size of the transistor creates several issues. For example, the electrical resistance of the gate electrode is increased when the gate electrode is reduced. This causes the transmission speed of an electrical signal applied to the gate electrode to slow down due to the resistance-capacitance (RC) delay time. Additionally, a short channel effect is created due to the reduced channel length. In order to minimize the short channel effect, the source and drain junction depth should be fabricated to be shallow and the gate insulating layer should be fabricated to be thin.
In this case, when the PMOS gate electrode 10′ is fabricated to be thin and the PMOS gate insulating layer 8 is also fabricated to be thin, problems such as polysilicon depletion and boron penetration become more severe. This occurs because the solid solubility of boron (B) with respect to the polysilicon is typically low. Accordingly, the boron ions implanted into the PMOS gate electrode 10′ penetrate the PMOS gate insulating layer 8 (which has been fabricated to be thin), thereby diffusing into a channel region of the PMOS transistor. When the boron penetration phenomenon becomes severe, a depletion region is formed within the PMOS gate electrode 10′ near the PMOS gate insulating layer 8. The polysilicon depletion region generates additional capacitance which is connected in series to the capacitance of the PMOS gate insulating layer 8. As a result, the polysilicon depletion region increases the electrical equivalent thickness of the PMOS gate insulating layer 8. The increase of the electrical equivalent thickness means a decrease in the effective gate voltage. In the prior art using a thick gate insulating layer, the thickness of the polysilicon depletion region is relatively small compared to the effective thickness of the thick gate insulating layer, so that its effect is negligible. However, when a thin gate insulating layer is used, the decrease of the effective gate voltage due to polysilicon depletion creates a severe problem. In addition, the boron (B) ions diffused into the channel region of the PMOS transistor may lower the mobility of the charge carrier in the channel region and may form P-type impurity layers connecting the source to the drain within the channel region. This would make it hard to adjust the operating voltage Vt.
A method of forming a CMOS type semiconductor device using the dual gates is disclosed in U.S. Pat. No. 6,166,413 entitled “Semiconductor device having field effect transistors different in thickness of gate electrodes and process of fabrication thereof” to Ono.
According to Ono, un-doped polysilicon patterns are formed in NMOS and PMOS transistor regions, respectively. The un-doped polysilicon pattern in the NMOS transistor region is over etched to have a small thickness. Arsenic (As) ions are selectively implanted into the NMOS transistor region to form an N-type gate electrode and N-type source and drain regions. Subsequently, B ions are implanted into the PMOS transistor form a P-type gate electrode and P-type source and drain regions. As a result, the N-type gate electrode thickness is smaller than that of the P-type gate electrode. However, the P-type gate electrode is concurrently formed while the P-type source and drain regions are formed. When the P-type gate electrode and the P-type source and drain regions are concurrently formed, it is difficult to control the amount of implantation of the B ions. That is, when an excessive amount of B ions are implanted into all of the P-type gate electrode and the P-type source and drain regions, the boron penetration phenomenon may become severe. On the contrary, when an insufficient amount of B ions are implanted into all of the P-type gate electrode and the P-type source and drain regions, the junction characteristic of the P-type source and drain regions may be degraded.
In conclusion, continuing improvement is required for a technique of forming the P-type gate electrode and the N-type gate electrode.